The present invention relates to medical electrodes and leads generally, and more particularly to implantable defibrillation electrodes and leads.
By way of definition, in the field of automatic implantable arrhythmia control devices, the term "cardioversion" or "cardioverter" refers to the process of and device for discharging relatively high energy electrical pulses into or across cardiac tissue to arrest a life threatening tachyarrhythmia. Cardioversion pulses may or may not be synchronized with a cardiac depolarization or rhythm and may be applied to arrest a ventricular tachycardia with a lower range energy pulse or ventricular fibrillation with a medium to high energy pulse. The arrest of ventricular fibrillation by such pulses is referred to as "defibrillation", a form of cardioversion, and "defibrillators" have been characterized as a form of cardioverter. In the following description and claims, it is to be assumed that these terms are interchangeable, and that use of one term is inclusive of the other device or operation, unless specific distinctions are drawn between them in the context of the use.
Early concepts of implantable defibrillators, such as disclosed in U.S. Pat. No. Re. 27,652 by Mirowski et al., envisioned an electrode system employing a ventricular endocardial electrode and a plate electrode mounted to the heart or implanted subcutaneously. However, early commercially available implantable defibrillators typically employed multiple epicardial electrodes, the surgical implantation of which required a thoracotomy.
More recent implantable defibrillators employ systems of endocardial and optionally subcutaneous electrodes which avoid the necessity of a thoracotomy. Electrode systems of this type are disclosed in U.S. Pat. No. 4,727,877 issued to Kallok, U.S. Pat. No. 4,708,145 issued to Tacker et al., U.S. Pat. No. 4,922,927, issued to Fine et al., U.S. Pat. No. 5,261,400 issued to Bardy and U.S. Pat. No. 5,099,838 issued to Bardy, all incorporated herein by reference in their entireties.
The subcutaneous leads employed in the systems as discussed above may be fabricated using metal mesh electrodes, as disclosed in U.S. Pat. No. 4,765,341, issued to Mower et al, or coiled metal wire electrodes as disclosed in commonly assigned U.S. Pat. Nos. 4,817,634, issued to Holleman et al., and 5,044,374, issued to Lindemans, or may be the external surface of the metal enclosure of the defibrillator pulse generator as disclosed in the above-cited Kallok, Bardy '400 and Fine patents.
In U.S. Pat. No. 5,133,353, issued to Hauser, a "mesh" electrode formed on a portion of the pulse generator case is employed as one electrode in a two or three electrode system. It is suggested that a further conductive patch electrode may be attached by means of an attachment mechanism to the pulse generator housing to increase the conductive surface area.
Further efforts to enhance efficacy and decrease cardioversion/defibrillation efficiency have led to the suggestion of a wide variety of pulse wave forms and polarities for use as cardioversion/defibrillation shock pulses. Monophasic capacitive discharge pulses are disclosed in the above cited Mirowski reissue patent and were employed in the earliest devices implanted in patients. Biphasic pulses are disclosed in U.S. Pat. No. 4,953,551, issued to Mehra et al. Damped sinusoidal pulses are disclosed in U.S. Pat. No. 4,834,100, issued to Charms.
In spite of the gains in efficiency provided by the developments disclosed in the patents discussed above, efforts are presently underway to further reduce the size of current implantable defibrillator pulse generators to further simplify implant and enhance patient comfort. As the devices become smaller, it is anticipated that the surface areas of the defibrillator housings may become small enough to interfere ability of the housing to function efficiently as the subcutaneous defibrillation electrode.